Process transmitters are used to monitor process variables, such as pressure, temperature, flow and level of process fluids used in a variety of industrial processes. In some instances, process transmitters to measure pressure and/or flow parameters can include an isolating diaphragm that is coupled to industrial process equipment, such as a pipe, a reservoir, or other process equipment. Generally, the isolating diaphragm seals at least a portion of a process transmitter housing from exposure to the process fluids. Pressure measurements may be taken directly from the diaphragm based on the diaphragms deflection in response to pressure or may be taken indirectly by a remote pressure sensor that is coupled to the isolating diaphragm by a fluid filled capillary. Such pressure measurements may be used directly or may be used to calculate a fluid flow rate, for example.
Conventionally, a process transmitter housing is formed from a corrosion resistant material, such as stainless steel. The diaphragm is typically a one-piece diaphragm including an outer ring portion and a deformable diaphragm portion, which are typically formed from Tantalum or another corrosion resistant material. To join the diaphragm structure to the process transmitter housing, a brazing process is performed, which includes applying a brazing material between the outer ring of the diaphragm and the transmitter housing. The diaphragm structure and the transmitter housing are heated and cooled to join the dissimilar metals. Unfortunately, the different materials of the diaphragm structure and the transmitter housing have different coefficients of thermal expansion. The brazing process may cause buckling and other defects to form in the diaphragm structure as the different metals expand and contract at different rates.
Tantalum can be especially difficult to braze in this configuration because the coefficient of thermal expansion for Tantalum under temperature is significantly different than the coefficient of thermal expansion of the stainless steel transmitter housing. Upon cooling, the stainless steel contracts about three times more than the Tantalum diaphragm structure. As the structure cools, the tantalum diaphragm structure can buckle, causing visible damage. In some instances, such buckling may not be visibly detectable, but may nevertheless result in variability in the performance of the diaphragm structure. Traditionally, the diaphragm structure included features in its shape to overcome such defects; however, performance of individual pressure transmitters could be inconsistent. Hence, there is a need for an improved diaphragm structure and method of manufacturing a diaphragm structure.